1. Field of the Invention
The invention relates to an exhaust purifying catalyst, and more particularly to an exhaust purifying catalyst which, by using a specific combination of oxygen storage/release capacity materials (also abbreviated below as “OSC materials”) in the catalyst, is able to provide a high NOx purifying capacity even following endurance under fluctuating air-to-fuel (A/F) ratio conditions. In this specification, “high NO purifying capacity” refers to a NOx purifying capacity which is comparable to or higher than that of conventional exhaust purifying catalysts.
2. Description of the Related Art
The exhaust gases discharged from automotive and other internal combustion engines contain HC, CO and NOx. These substances are cleansed with an exhaust purifying catalyst, then released into the atmosphere. Three-way catalysts composed of a noble metal such as platinum (Pt), palladium (Pd) or rhodium (Rh) supported on a porous oxide support such as alumina Al2O3), (silica (SiO2), zirconia (ZrO2) or titania (TiO2) are widely used as typical exhaust purifying catalysts.
Such a three-way catalyst purifies by oxidizing the HC and CO in exhaust gases and reducing NOx and is most highly effective for cleansing exhaust gases from a stoichiometric atmospheric that has been combusted near a stoichiometric air-fuel ratio. Recently, in particular, given the desire to increase gas mileage, modifications such as raising the fuel cut (FC) number at high temperature have increased the opportunities for exposure of the exhaust purifying catalyst to rapid atmospheric fluctuations based on fluctuations in the A/F ratio at high temperature. Such rapid atmospheric fluctuations greatly accelerate catalyst deterioration.
In an exhaust purifying catalyst, the A/F ratio of gas entering the catalyst constantly undergoes large changes due to driving conditions such as acceleration and deceleration. In general, the catalyst interior is controlled so as to be stoichiometric by an oxygen sensor placed downstream from the exhaust purifying catalyst. Hence, there is a desire for the catalyst to have a rapid oxygen storage/release rate which can absorb excessive changes in the A/F ratio. At the same time, there also exist in the drive mode drive regions where changes in the vehicle velocity are sluggish, and so there also is a desire for the ability to manifest a long-term oxygen storage capacity in circumstances where changes in the A/F ratio of gas entering the catalyst are slow.
In addition, because the noble metals used in exhaust purifying catalysts are expensive and pose certain challenges in terms of resources, there is a need to increase the catalyst performance and reduce the amount in which such metals are used. At the same time, there is a desire to conform to emissions standards which are becoming increasingly stringent. Because of such concerns, various investigations are being conducted with the aim of increasing the activity of exhaust purifying catalysts.
For example, Japanese Patent Application Publication No. 2008-62130 (JP-A-2008-62130) describes an exhaust purifying catalyst which contains oxygen storage material particles. The catalyst is obtained by mixing first cerium-based oxygen storage material particles (A) having a first number-mean particle diameter and second cerium-based oxygen storage material particles (B) of the same composition as the above particles (A) and having a second number-mean particle diameter that is larger than the first number-mean particle diameter so as to adjust the resulting oxygen storage capacity deterioration time to a predetermined time. However, this publication mentions nothing about the catalytic performance of the catalyst.
International Publication No. 2008-93471 discloses a catalyst system for use in an automotive exhaust purifying device, the catalyst system using two or more exhaust purifying catalysts which include a first catalyst supported on a support having an inorganic structure and a second catalyst which differs from the first catalyst. The first catalyst is supported at an inorganic structure supporting portion positioned on the upstream side in an exhaust flow channel, and the second catalyst is supported at an inorganic structure supporting portion positioned on the downstream side in the exhaust flow channel and includes in the crystal structure a pyrochlore phase-containing cerium-zirconium-based complex oxide (A). The results given in the specific examples are an air excess ratio λ [(actual air/fuel ratio)/(stoichiometric air/fuel ratio)] of 1.02 or more and a decrease in the NOx purification ratio to below 60%.
In addition, Japanese Patent Application Publication No. 2009-19537 (JP-A-2009-19537) discloses an exhaust purifying catalyst having Pd/alumina composed of palladium supported on alumina particles and Rh/OSC material composed of rhodium supported on oxygen storage material particles disposed at an upstream side catalyst portion, and having Pt-alumina composed of platinum supported on alumina particles and Rh/OSC material composed of rhodium supported on oxygen storage material particles disposed at a downstream side catalyst portion. The oxygen storage material particles at the upstream side catalyst portion are composed of a complex oxide having a weight ratio ZrO2/CeO2 of 1 or more. However, no mention whatsoever is made of catalyst containing free OSC material in which an OSC material does not support a noble metal.
Because the exhaust purifying catalysts described in these patent publications do not have a sufficient NOx purifying capacity following endurance under fluctuating A/F ratio conditions, there exists a desire for an exhaust purifying catalyst having a higher NOx purifying capacity.